1. SUMMARY OF THE INVENTION
The present invention provides an improved system for lowering the emissions of nitrogen oxides generated by the combustion of hydrocarbons. More specifically, the present invention provides a system wherein anhydrous ammonia is present during the combustion of the hydrocarbons thereby lowering the emissions of nitrogen oxides. The present invention additionally provides a method for lowering emissions of nitrogen oxides generated by the combustion of hydrocarbons.
2. BACKGROUND OF THE INVENTION
Oxides of nitrogen (nitrogen oxides, NO.sub.x) are one of five primary pollutants generated by the combustion of hydrocarbons (the others are oxides of sulfur, carbon monoxide, hydrocarbons and particulates). While there are several oxides of nitrogen, nitric oxide (NO) and nitrogen dioxide (NO.sub.2) are the principal air pollutants. It is interesting to note that the hydrocarbons provide neither the nitrogen nor the oxygen of the NO.sub.x. Rather, the nitrogen and oxygen present in the air, which supports the combustion of the hydrocarbons, react due to the high temperatures of combustion to form NO. The NO then reacts immediately with O.sub.2 to produce NO.sub.2.
In areas such as Los Angeles, Buenos Aires and Mexico City, the accumulation of NO.sub.2 results in the formation of photochemical smog, which is a product of the interaction of NO.sub.2, O.sub.2, hydrocarbons and ultra-violet light from the sun. Beyond the undesirable haze which results from this smog, the citizens of these regions may experience breathing difficulties, watery eyes and nasal irritations. In general, the tolerance level for NO.sub.2 in air is set at 3 ppm.
While the majority of NO produced by land based engines reacts with oxygen in the air to produce NO.sub.2, the NO produced by aircraft while in flight may have a detrimental effect on the ozone layer. This is shown by the following reactions: EQU NO+O.sub.3 .fwdarw.NO.sub.2 +O.sub.2 EQU O.sub.3 .fwdarw.O.sub.2 +O EQU NO.sub.2 +O.fwdarw.NO+O.sub.2
These reactions yield a net reaction of 2O.sub.3 .fwdarw.3O.sub.2, which demonstrates the possible depletion of the ozone layer.
Previous attempts have been made to lower the emissions of NO.sub.x. One method utilized in automobiles is the exhaust gas recirculation system. This method returns a portion of the exhaust gas to the cylinders lowering the emissions of NO.sub.x by reducing the flame temperature in the cylinder. However, this method also reduces the efficiency of the engine. Another method for lowering emissions of NO.sub.x provides for the injection of anhydrous ammonia into the exhaust system of an engine. The ammonia is injected immediately upstream of the exhaust system's catalytic converter and reacts with the NO.sub.x thereby lowering emissions of NO.sub.x. The catalytic converter increases the rate of this reaction.
Although progress has been made in lowering the emissions of all pollutants, the additional lowering of all NO.sub.x emissions is desirable. Therefore, one object of the present invention is to provide an improved emissions system for lowering the emissions of all NO.sub.x generated by the combustion of hydrocarbons.